The present invention relates to steam generators of the kind utilized in power plants and having steam superheating facilities. More particularly, the invention involves method and apparatus for regulating superheated steam temperatures of the steam generator especially during startup of the unit.
In the operation of a power plant system some steam generators are arranged to accommodate the base loading of the system while others satisfy fluctuating loads such as may be attendant with nightly or weekend needs of the system. Steam generators which are base loaded are adapted for substantially continuous, uniform operation while those that are accommodating the fluctuating portion of the system demand must be designed for cycling service. This latter type of operation subjects the steam generator and turbine to significant stresses, induced primarily by the temperature differences between components that are caused by cyclic operation.
It is known that cyclic stresses of this type can be ameliorated by limiting the temperature differences that result from cycling service of the unit. Superheating steam generators in the past have been operated in a fashion and have incorporated means to so limit steam temperature differences. Such means have involved supplementary fluid circuits that bypass some of the main operating fluid around the superheater, discharging the same to the condenser. Others incorporate means for venting the steam drum, while still others involve means for blowing down the evaporator. In all of the above instances the quantity of steam passed to the superheater is reduced thereby resulting in increased steam temperatures at the superheater outlet. Such steam temperatures, however, are increased at the cost of significant amounts of working fluid and the heat contained therein being rejected from the system thereby reducing the operational efficiency of the plant.
It is to the improvement, therefore, of steam generator flow systems and their methods of operation that the present invention is directed.